Algorithm 3: Pseudocode of Ext()

Input: an affected carrier message (CM’HM), a secret key (K) Output: a secret message (SM’) 1. HS← Discovers the existing hidden marks/symbols from the CM’HM; 2. K← Secret Key (e.g., the symmetric or asymmetric key algorithm such as One-Time-Pad, AES, DES, etc.); 3. for each $c_i\in HS=\left\{c_1,c_2,\dots ,c_n\right\}$ do 4. Encrypted_SMbits← Encrypted_SMbits + Detects the binary string of each invisible symbol from $HS[l_i]$; 5. Kbits←Kbits+ Utilizes a shared key from Alice or Extracts the secret key from the CM’HM. 6. end for 7. SMbits← Decrypts the Encrypted _SMbits based on Kbits using corresponding decryption function; 8. SM’← Extracts the original SM characters from the SMbits based on their ASCII codes. 9. Return SM’;